This invention relates to methods for clarifying coal liquors or slurries by employing water-soluble polymers.
Clarification of coal liquors or slurries is an important aspect in the economy of operating a coal production plant. Generally, such plants are operated on a closed system basis in that the water used to wash the raw coal is recycled for reuse. Since such water serves primarily to remove coal fines and clays mixed with the raw coal thereby forming coal liquors, it is critical that the water be relatively clear and contain only minor amounts of suspended solids when it is reused in the washing procedure. Otherwise, undesirable large volumes of water would be necessary to process the coal. Since the coal fines and clay which are removed from these coal liquors are often used as filler materials and in other applications which require inexpensive inert materials, it is essential that such coal liquor clarification be very inexpensive.
Previously, the coal industry has employed various anionic and cationic, water-soluble polymers to remove both coal particles and colloidal clay particles. For example, as taught in U.S. Patent No. 3,408,293, it is a common practice to prefloc the coal liquor or coal slurry with anionic polymers in order to flocculate the coal particles and subsequently treat the prefloc slurry with cationic polymer in order to coagulate the remaining clay and/or coal particles. Unfortunately, very careful control over the quantities of anionic and cationic polymers employed must be exercised in order to obtain efficient clarification of the coal liquor. Also, significant quantities of the various polymers need to be employed in order to achieve such desirable coal liquor clarification.
Colloidal coal particles in aqueous alkaline medium are flocculated by the combined action of a low molecular weight water-soluble anionic polymer which contains at least 50 mole percent acrylic acid linkages, and a high molecular weight anionic polymer containing at least 95 mole percent acrylamide, as disclosed in U.S. Patent No. 3,717,574. Water-soluble anionic starches are also disclosed to be supplementary flocculants. Unfortunately, the treatment of a coal slurry with such a polymer system can result in a partially flocculated slurry having unflocculated coal particles suspended in the aqueous medium.
In view of the foregoing deficiencies of the prior art procedures for coal liquor clarification, it is highly desirable to provide a method which allows for effective and efficient flocculation causing removal of substantially all collidal coal from the aqueous media while employing a relatively low flocculant dosage.